Medical devices may be coated so that the surfaces of such devices have desired properties or effects. For example, it may be useful to coat medical devices to provide for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Localized drug delivery may avoid some of the problems of systemic drug administration, which may be accompanied by unwanted effects on parts of the body which are not to be treated. Additionally, treatment of the afflicted part of the body may require a high concentration of therapeutic agent that may not be achievable by systemic administration. Localized drug delivery may be achieved, for example, by coating balloon catheters, stents and the like with the therapeutic agent to be locally delivered. The coating on medical devices may provide for controlled release, which may include long-term or sustained release, of a bioactive material.
Aside from facilitating localized drug delivery, medical devices may be coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow for fluoroscopic visualization during placement in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
Coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electrodeposition. Although these processes have been used to produce satisfactory coatings, they have numerous, associated potential drawbacks. For example, it may be difficult to achieve coatings of uniform thicknesses, both on individual parts and on batches of parts. Also, these coating processes may require that the coated part be held during coating, which may result in defects such as bare spots where the part was held and which may thus require subsequent coating steps. Further, many conventional processes require multiple coating steps or stages for the application of a second coating material, or to allow for drying between coating steps or after the final coating step.
There is, therefore, a need for a cost-effective method of coating medical devices that results in uniform, defect-free coatings and uniform drug doses per unit device. The method would allow for a multiple stage coating in order to apply a bioactive material that may be environmentally sensitive, e.g., due to heat and light (including ultra-violet) exposure. Multiple stage coating may also be used to prevent degradation of the bioactive material due to process-related forces (e.g., shear). The method would thus allow for better control of the sensitivity of the bioactive material and reduce any potential degradation due to environmental issues. The method would also reduce variations in the coating properties.
Gas suspension coating is a process by which a large number of stents may be freely suspended in a nitrogen (or other) gas stream as a coating is applied and dried in one process. Two of the issues facing gas suspension are the effect of the process on the mechanical integrity of the stent and the effect on the coating.
One cause of damage to the stent and the coating in a gas suspension coating process is the velocity of the stent as it is fluidized. A gas suspension system utilizes a gas atomizing spray nozzle, which uses a jet of gas that can shoot stents at rapid speeds and can cause both coating and stent damage. This velocity can damage a stent and coating as the stent impacts other stents and the inside of the vessel. If this pressure could be reduced or eliminated then damage would be reduced or eliminated. These issues may become more critical with more flexible stent designs.
Current state of the art for gas suspension involves the use of a gas atomizing spray nozzle mounted at the base of a hurricade vessel. In order to obtain efficient coating it is necessary to mount the nozzle at the base to maximize the transfer of coating to the stents. One issue with mounting the spray nozzle at the base may be that the direction of the spray may be straight up. This direction is the same as that of the fluidization gas which suspends the stents. The atomization pressure may be highly focused and may tend to shoot the stents with high velocity when they pass over the spray nozzle.
There thus is a need for a method of coating stents in a gas suspension system that does not cause damage to the coating or stents.